Railroad trains in North America, Europe, and substantial parts of the world are equipped with some form of automatic pneumatic brake systems, sometimes referred to as air brakes. This strictly pneumatic brake system provides a simple, reliable, and generally fail-safe means for permitting the engineer, conductor, or train crew to apply the brakes throughout the train as well on the locomotive. The automatic pneumatic brake system is a continuous power brake system having an air compressor on the locomotive connected to a brake pipe extending throughout the train. An automatic brake valve is located on the locomotive which the engineer uses to reduce or increase air pressure in the brake pipe. The standard automatic brake valve has a release position, an initial reduction position, a service braking zone, a suppression position, a handle off position, and an emergency position. Each car of the train has a control valve which senses a "reduction" or "increase" of air pressure in the brake pipe initiated by the engineer at the automatic brake valve, and applies or releases the brakes according to the "reduction" or "increase" command, respectively. The control valves vary in construction and in operating features to suit freight or passenger trains.
While air brakes are used on both freight and passenger trains, the demands on each system are quite different due to the length of the train, the weight of the train, the speed of the train, and other various factors. The length of the train is especially important since air pressure reductions in the brake pipe travel at approximately the speed of sound. In a long freight train, such as one having one hundred fifty cars and a possible length of one and one-half miles, it takes approximately eighteen seconds for the air pressure reduction initiated in the locomotive to reach the last car in the train. Accordingly, in the prior known automatic pneumatic brake systems on freight trains, the brake cylinder pressure build-up time has to be carefully retarded in the front cars to prevent the last cars of the train, where the brakes have not yet been applied due to the signal delay, from running into the front cars with fully developed brake cylinder pressure. Consequently, full pressure braking is delayed and braking distances are longer. On shorter trains such as passenger trains, this is not such a significant problem, even though there is some delay between the braking of the first and last cars.
To solve these problems, electronically controlled pneumatic brake systems have been proposed and are currently being tested. For example, Technical Services & Marketing, Inc. has proposed and is testing a retrofit electronic air brake system disclosed in U.S. Pat. No. 5,355,974. The electronically controlled pneumatic brake system generally incorporates part of the automatic pneumatic brake system equipment including the brake pipe, the reservoir tanks, the brake cylinder(s), and the rigging or linkage between the brake cylinder and the brakes. Control of the brake cylinder pressure is accomplished through a computer controlled network wherein each car is equipped with a control device and the locomotive is equipped with a head end unit or a master controller. Generally, in the prior known electronically controlled pneumatic brake system, the head end unit determines the amount of brake cylinder pressure and sends a corresponding signal, including the amount of brake cylinder pressure, to each controller. The car control devices act through solenoid controlled valves to fill and exhaust the brake cylinder in response to the electronic brake cylinder pressure signals issued by the head end unit. The brake cylinder pressure on each car is thereby directly controlled by the head end unit in response to an electronic braking or release command from the engineer. The electronically controlled pneumatic brake system provides substantially instantaneous and simultaneous brake signals to all of the cars which enables all of the cars to brake at substantially the same time (i.e., increasing the rate of brake cylinder pressure build-up). The electronically controlled pneumatic brake system also provides the possibility of a graduated release of the brakes since the brake cylinder pressure on each car is under virtually simultaneous control of the engineer. The brakes can be applied, partially released, then reapplied as often as necessary, within the ability of the locomotive air compressors to replenish reservoirs at the rear of a long train.
While the general concept of electronically controlled pneumatic brake systems has been proposed, there currently are no Association of American Railroads (AAR) approved systems which completely, adequately, and safely implement an electronically controlled pneumatic brake system installation on present passenger or freight trains. To foster development of electronic air brake systems, the AAR recently issued a report on electronically controlled pneumatic brake systems entitled "UPDATED STATUS OF AAR ELECTRIC BRAKE SPECIFICATION" to provide general guidelines for the industry in developing electronically controlled pneumatic brake systems according to minimum standards and specifications. Accordingly, there is a need for a fail-safe electronically controlled pneumatic brake system adapted to be installed and work in conjunction with the pneumatic braking rigging or equipment on present day freight cars that completely, adequately, and safely implements an electronically controlled pneumatic brake system and eliminates the problems of the old air brake systems.